This invention relates to an inverter type X-ray apparatus, and more particularly to a device effectively applicable to a high voltage generating device in an X-ray apparatus in which an inverter is connected to the input of a high voltage transformer.
A prior art inverter type X-ray apparatus comprises an inverter including four switching elements in a bridge connection, and the switching elements disposed opposite to each other are operated alternately to generate an AC voltage of rectangular waveform. This AC voltage of rectangular waveform is applied to a high voltage transformer to be transformed into a high AC voltage, and, after converting this high AC voltage into a DC voltage by a high voltage rectifier circuit, the DC voltage is applied to an X-ray tube, through a high voltage cable.
One form of such a prior art inverter type X-ray apparatus is shown in FIG. 4.
Referring to FIG. 4, an AC power supply voltage from a commercial AC power source 1 is rectified by a full-wave rectifier circuit I composed of rectifiers 2, 3, 4 and 5, and the output of the full-wave rectifier circuit I is smoothed by a smoothing circuit composed of a winding 6 and a capacitor 7 to provide a DC voltage including a small proportion of ripple components. The output of the smoothing circuit is applied to a chopping circuit II composed of a transistor 8, a transistor drive circuit 9, a free-wheel diode 10, a winding 11, a capacitor 12, and voltage detecting resistors 13 and 14. The ratio between the on-period and the off-period or what is called herein a duty cycle of the switching operation of the transistor 8 is changed depending on a pre-set condition of X-ray exposure thereby regulating the DC output voltage of the chopping circuit II. The output of the chopping circuit II is applied to a full-bridge type inverter circuit III composed of transistor 15, 16, 17, 18, free-wheel diodes 19, 20, 21, 22, transistor drive circuits 23, 24, 25, 26, and an inverter control circuit 41. The DC output voltage of the chopping circuit II is inverted by the inverter circuit III into a high-frequency AC voltage having usually a frequency as high as several hundred Hz, and this AC voltage is applied to a high voltage transformer 27. The output voltage of the high voltage transformer 27 is converted into a DC voltage by a high voltage rectifier circuit composed of rectifiers 28, 29, 30 and 31 and is then applied to an X-ray tube 34 through a high voltage cable. Reference numerals 32 and 33 designate electrostatic capacitances of the high voltage cable. A tube voltage signal EKv proportional to a tube voltage, setting is applied to a tube voltage signal input terminal 38, and a tube current signal EmA proportional to a tube current setting is applied to a tube current signal input terminal 39. An X-ray exposure start signal Exp is applied to an X-ray exposure start signal input terminal 40.
In the prior art X-ray apparatus having such a construction, a tube voltage kV applied to the X-ray tube 34 is regulated by controlling the duty cycle of the chopping operation of the chopping circuit II. A signal for controlling the duty cycle of the chopping operation of the chopping circuit II is produced in accordance with the tube voltage signal Ekv and the tube current signal EmA. Describing this more concretely, a monostable multivibrator 36 connected to an oscillator 35 generates a duty-cycle pulse signal. This pulse signal is amplified by the transistor drive circuit 9 for switching the switching transistor 8, and the output voltage of the chopping circuit II is detected by the voltage detecting resistors 13 and 14. The detected voltage is fed back to a comparator 37 so as to be compared with the duty-cycle controlling signal. Therefore, a stabilized output voltage can be always generated regardless of a variation of the AC power supply voltage applied from the AC power source 1 and, also, regardless of, a load variation occurring when the inverter operates under various load conditions.
Suppose now that the inverter control circuit 41 generates on-pulses each lasting over the half period of inverter operation as shown at (a) and (b) in FIG. 5. In such a case, an input voltage V.sub.1 applied to the high voltage transformer 27 has a flat AC voltage waveform having the same peak value, as shown at (c) in FIG. 5. Such an input voltage V.sub.1 is applied to the primary winding of the high voltage transformer 27, and the resultant output voltage appearing across the secondary winding of the high voltage transformer 27 is rectified into a DC voltage by the high voltage rectifier circuit composed of the rectifiers 28, 29, 30 and 31. An overshoot Pm as shown at (d) in FIG. 5 appears in the AC-DC rectified tube voltage waveform immediately after the application of the input voltage. Further, undesirable transition tends to occur in the waveform each time the input pulse is applied.
The overshoot Pm and undesirable transition described above tend to increase the undesirable ripple of the tube voltage, thereby lowering the X-ray output and degrading the accuracy of the tube voltage.
Factors giving rise to these problems include a leakage inductance of the high voltage transformer 27, an electrostatic capacitance present between the shield and the core of the high voltage cable and an internal impedance of the X-ray tube 34. These factors are considered to provide the source of undesirable ripple.